1. Field of the Invention
The present invention relates to a combustion control apparatus for an engine, and more particularly to ignition timing control for performing fuel injection by directly injecting fuel into the combustion chamber in a cylinder with an injector, so as to generate a premixed air-fuel mixture which causes self ignition by compression.
2. Description of the Related Art
Generally, a direct-injection diesel engine injects fuel into a combustion chamber at a high temperature and high pressure near the top-dead-center position of a compression stroke in a cylinder so as to cause self ignition of the fuel. At this time, the fuel injected into the combustion chamber progresses while being split into fine droplets (atomized) by collision with highly dense air, so as to form an approximately cone-shaped fuel spray. The fuel droplets vaporize from its surface and involve surrounding air mainly at the leading edge and its periphery of the fuel spray to form a mixture which starts combustion at the timing when the density and temperature of the mixture attains the condition required for ignition, i.e., premixed combustion. Then, the combustion shifts to diffusion combustion involving surrounding fuel vapor and air, with at its core the ignition or combustion which has firstly occurred in the above mentioned manner.
In such combustion of a conventional diesel engine (herein referred to as diesel combustion), the major part of fuel causes the diffusion combustion following the initial premixed combustion. At this time, however, in the fuel spray mixture which is heterogeneous in density, nitrogen oxide (NOx) is produced by the abrupt heat production at the portion where excess the air ratio λ close to 1. Moreover, soot is produced by the shortage of oxygen at the portion where the fuel is unduly rich. In this regard, conventionally, the recirculation of part of the exhaust gas to intake air, i.e., exhaust gas recirculation (EGR) or the boosting of fuel injection pressure are put into practice in order to reduce NOx and soot.
During such EGR, the recirculation of the inactive exhaust gas decreases the combustion temperature to suppress the generation of NOx, but on the other hand, reduces the amount of oxygen in the intake air. Thus, a large amount of EGR results in the promotion of soot production. In addition, the boosted fuel injection pressure promotes atomization of fuel spray and increases fuel penetration to improve the air-utilization ratio, which is capable of suppressing the generation of soot, but is likely to easily generate NOx. That is, because of the trade-off relationship between the reductions in NOx and soot, it is actually difficult to decrease both NOx and soot simultaneously during diesel combustion.
To address this problem, a new combustion concept has recently been proposed, which significantly and concurrently reduces NOx and soot by greatly advancing the fuel injection timing to attain a combustion condition mainly dominated by the premixed combustion. The combustion concept is generally known as a premixed compressive ignition combustion. Japanese publication of Patent Application No. 2000-110669 discloses a diesel engine that recirculates a considerable amount of exhaust gas during EGR and injects fuel at the timing within the compression stroke of a cylinder. The injected fuel sufficiently mixes with air to form the mixture, which self-ignites and combusts at the end of the compression stroke.
When such premixed combustion (the premixed compressive ignition combustion) occurs, the ratio of the exhaust gas returned to the intake air by the EGR (the EGR ratio) is increased by a certain amount from that in the diesel combustion described above. Especially, the exhaust gas of which heat capacity is larger than air is mixed with the intake air, and the density of fuel and air in the premixture is decreased to prolong a ignition delay time for sufficiently mixing fuel and intake air, (air and exhaust gas). In addition, the ignition timing of the premixture is generated in such a manner it is delayed to a near top-dead-center (TDC) position of the compression stroke, so as to achieve a heat generation characteristic with a high heat efficiency. Moreover, when the premixture ignites in the abovementioned manner, the inactive exhaust gas is substantially homogeneously diffused around the fuel and air. This absorbs the combustion heat, thereby greatly suppressing NOx generation.
For recirculating such a large amount of exhaust gas to combustion chambers of the respective cylinders, the conventional diesel engine described above is equipped with an exhaust gas recirculation passage having a large diameter communicating the intake passage with the exhaust passage. The exhaust gas is drawn from the exhaust passage upstream of a compressor of a turbocharger and is recirculated to an intake passage downstream of the compressor of the turbocharger. Furthermore, a regulator valve is provided for adjusting the amount of the exhaust gas flowing through the exhaust gas recirculation passage to achieve a proper ratio of the exhaust gas recirculation in the intake air.
However, in the case that the regulator valve adjusts the amount of the exhaust gas through the exhaust gas recirculation passage as described above, the recirculation amount of the exhaust gas does not immediately change upon the adjusting of the opening degree of the exhaust gas recirculation regulator valve, but changes after a lag time. Thus, for example, in the case of an increase in the flow amount of intake air caused by a rise in engine rotational speed, the recirculation amount changes after a lag time, which causes a problem wherein the EGR ratio is temporarily lowered so as to deviate from the proper range. Moreover, the amount of the exhaust gas remaining in the combustion chamber, so called internal EGR, changes depending on an engine operational condition which causes the EGR ratio to fluctuate.
Furthermore, even with the same EGR ratio, the change in temperature condition of the recirculating exhaust gas causes the ignition delay time to vary. That is, the ignition delay time is shortened with an increase in the recirculating exhaust gas temperature, in contrast, the ignition delay time is prolonged with a decrease in recirculating exhaust gas temperature. In addition, the change in temperatures of the combustion chamber and intake air cause the ignition delay time to vary.
Therefore, in the premixed compressive ignition combustion described above, merely adjusting the opening degree of the regulator valve in the exhaust gas recirculation passage is insufficient to maintain the ignition timing of the premixture constantly near the top-dead-center, which causes the problem that the optimum heat generation characteristic is not always attained.
Here, Japanese Publication of Patent Application Publication No. 2000-008929 discloses a control process of the ignition timing of the premixture. According to the process, a part of fuel corresponding to a required engine torque is injected into the combustion chamber at a time within a period from the intake stroke to the compression stroke, to form a relatively lean premixture. Then, the remaining part of fuel is injected near the top-dead-center position of the compression stroke to immediately cause diffusion combustion, which triggers the combustion of the premixture. However, the premixture is compulsorily forced to ignite by the diffusion combustion of the fuel injected at a later time, which causes problems of a considerable amount of soot generation during the combustion; and the degradation in fuel efficiency by a likely increase in the amount of the unburned mixture.
Reference may be made to a paper entitled “Development of Ignition Timing Control in HCCI DI Diesel Engine” by Yanagihara et al, Proceedings of JSAE No. 51-01, No. 20015025, Pages 17–22, May 2001.
The paper discloses a technology, in which the engine with a relatively low compression ratio injects so small an amount of fuel as not to ignite by itself at an early timing (for example, BTDC 50 degrees CA.) of the compression stroke in the cylinder, so as to generate premixture in the combustion chamber.
Then, while a low temperature oxidation reaction (a cool flame reaction) is continuing during the expansion stroke in which the temperature gradually lowers past the top-dead-center of the compression stroke of the cylinder, fuel is additionally injected to ignite and combust.
However, in the prior art, the additional fuel injection also triggers self-ignition. The difference of this prior art from the former prior art (Japanese Publication of Patent Application Publication No. 2000-008929) is that fuel injection timing on the relatively retarded side in tile expansion stroke (for example, ATDC 10 degree CA or after) of the cylinder is set for preventing the additional fuel injection from causing the diffusion combustion. Thus, the greatly retarded ignition timing causes the cycle efficiency to decrease and the amount of unburned premixture to increase, which significantly degrade the fuel efficiency.